Lithophile and siderophile element systematics of Earth’s mantle at the Archean- Proterozoic boundary: Evidence from 2.4 Ga komatiites

Abstract

New Os isotope and highly siderophile element (HSE) abundance data, in combination with lithophile trace element and Sm–Nd, Lu–Hf, and Hf–W isotope data, are reported for komatiitic basalts from the Vetreny Belt and tonalites from the adjacent Vodla Block in the Fennoscandian Shield. Komatiitic basalts define a Re–Os isochron with an age of 2407 ± 6 Ma and an initial γ187Os = +1.7 ± 0.2 (2 SE). The Pt–Os data for chromite separates yield an average initial ε186Os = +0.03 ± 0.02 (2 SE). The 147Sm–143Nd and 176Lu–176Hf data for the komatiitic basalts give isochron ages and initial ratios of, respectively, 2403 ± 32 Ma and ε143Nd = −0.90 ± 0.09, and 2451 ± 79 Ma and ε176Hf = +0.4 ± 0.2 (2 SE). Bulk tonalites are characterized by average initial γ187Os, ε143Nd, and ε176Hf values of +304 ± 64, +1.8 ± 0.6, and +2.5 ± 1.6 (2 SE), respectively, when calculated for the ∼3.21 Ga age of the rocks. The komatiitic basalts and tonalites have μ142Nd values of, respectively, +0.5 ± 2.8 and −0.4 ± 5.2 (2 SD). By contrast, both the komatiitic basalts and tonalites exhibit positive 182W anomalies of +7.1 ± 4.5 and +12.6 ± 4.5 ppm (2 SD), respectively.